1. Field of the Invention
This invention relates to a cathode material for manufacturing a rechargeable battery, more particularly to a cathode material for manufacturing a rechargeable battery which includes micrometer-sized secondary particles, each of which is composed of crystalline nanometer-sized primary particles. This invention also relates to a cathode for a rechargeable battery including the cathode material.
2. Description of the Related Art
Along with diversified development of electronic products, there is increasing need for portable power supplies. For example, electronic consumer products, medical devices, motorcycles, automobiles, and power tools and the like require a portable power supply for power source. For current portable power supplies, rechargeable batteries are relatively popular. Since lithium rechargeable batteries have a high ratio of volume to capacity, pollution-free and recyclable charge/discharge properties, and no memory effect, it has a great development potential in the future.
Additionally, a cathode material used for manufacture of the cathode plays an important role in the performance of the rechargeable battery. Among the known cathode materials, since lithium ferrous phosphate compounds and the related compounds having similar properties to LiFePO4 compounds, such as LiMPO4, in which M represents transition elements, e.g., manganese (Mn), cobalt (Co), and nickel (Ni), are environmentally benign, relatively stable and abundant, and have relatively good electrochemical properties, e.g., high specific capacity, good charge/discharge cycling performance, and good thermostability, they have been evaluated to be the cathode material with greatest development potential.
However, at present, there is a difference between practical and theoretical electrochemical properties of known LiFePO4 compounds and the related compounds. For example, the theoretical specific capacity of LiFePO4 compounds and the related compounds is about 170 mAh/g, whereas the LiFePO4 compounds disclosed in U.S. Pat. No. 5,910,382 have a specific capacity of about 95 mAh/g, which is far below the theoretical specific capacity. In order to improve the capacity property of the LiFePO4 compounds, it has been proposed to add other elements to the LiFePO4 compounds having one of olivine and NASICON structures so as to increase the capacity property of the LiFePO4 compounds, see U.S. Pat. Nos. 6,716,372 and 6,815,122. However, since the elements used for substituting iron are not easily available, production cost is relatively high.
In addition, U.S. Pat. No. 6,632,566 (hereinafter referred to as the '566 patent) discloses increase in the specific surface of the LiFePO4 compound powders in favor of diffusion of lithium ions in the powders, thereby enhancing capacity of a cathode material made from the LiFePO4 compound powders. Particularly, the cathode material described in the '566 patent is produced by sintering the LiFePO4 compound powders at a suitable temperature in such a manner that the cathode material thus formed is composed of separate single-phase crystalline particles having a grain size not larger than 10 μm. Although the capacity of the cathode material illustrated in the Examples of the '566 patent can be about 163 mAh/g, the LiFePO4 compound powders included in the cathode material have a relatively large particle size and uneven distribution. Thus, the cathode material of the '566 patent cannot be used with aqueous binders and aqueous solvents when applied to manufacture of the cathode. Besides, since the charge/discharge rate of the battery with the cathode material of the '566 patent is about C/37, which is calculated based on data shown in the Examples of the '566 patent, such charge/discharge rate is too low for practical application and needs to be improved.
The parent U.S. patent application Ser. No. 11/222,569 (hereinafter referred to as the '569 application) discloses a method for making a lithium mixed metal compound, such as the lithium ferrous phosphate compounds having an olivine structure. The method disclosed in the '569 application includes preparing a reactant mixture that comprises ion sources of the lithium ferrous phosphate compounds, and exposing the reactant mixture to a non-oxidizing atmosphere in the presence of suspended carbon particles. The lithium ferrous phosphate compounds thus made are in powder form and have relatively small particle size and even distribution.